Diaryl carbonates are useful as starting materials for the preparation of polycarbonates by melt processes. This melt process typically is conducted at temperatures up to 320.degree. C. in order to drive off aryl hydroxy compound and increase the molecular weight to useful levels. Small amounts (1-10 ppm) of basic ester exchange catalysts, such as alkali metal hydroxides, are commonly employed. An important advantage of this melt process over the solution process is that the polymer product melt may be directly extruded and chopped into pellets for sale. Isolation and purification from viscous solutions is not necessary, nor is the handling and recycling of large volumes of solvent.
U.S. Pat. No. 2,362,865 discloses the reaction of phenol and phosgene to form diphenyl carbonate. The reaction employed amphoteric metal catalysts and reaction temperatures from 150.degree. to 250.degree. C., preferably 180.degree. to 250.degree. C. The reaction is conducted in the melt without a solvent, i.e. under neat conditions. U.S. Pat. No. 3,251,873 discloses a similar process utilizing non-amphoteric metal catalysts and an organic solvent. Suitable reaction temperatures are from 50.degree. to 250.degree. C. depending on the reflux temperatures of the organic solvent at atmospheric pressure. U.S. Pat. No. 2,837,555 discloses ammonium halide catalysts, neat reaction conditions and reactor temperatures from 150.degree. to 250.degree. C. for a process similar to that of U.S. Pat. No. 3,251,873. U.S. Pat. No. 4,012,406 discloses the reaction of phenols and phosgene using heterocyclic basic nitrogen catalysts and temperatures from 25.degree. to 200.degree. C. in a gaseous reaction medium.